Electrostatic lens arrangement for cathode ray tubes



May 30, 1961 N. J. KoDA 2,986,669

ELECTROSTATIC LENS ARRANGEMENT FOR CATHODE RAY TUBES Filed Jan. 6, 1959 2 sheets-sheet 1 A T TOR/VE Y.

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N. J. KODA May 30, 1961 ELECTROSTATIC LENS ARRANGEMENT FOR CATHODE RAY TUBES Filed Jan. 6, 1959 2 Sheets-Sheet 2 Nobuo J. Kod.o,

BY. 11n/1,

ATTORNEY.

United States Patent ELECTROSTATIC LENS ARRANGEMENT FOR CATHODE RAY TUBES Nobuo J. Koda, Culver City, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Jan. 6, 1959, Ser. No. 785,162

7 Claims. (Cl. 313-86) This invention relates to direct-viewing cathode ray tubes of the type incorporating apparatus for producing an electron beam having a cross-sectional area conforming to the configuration of a desired information character and for reproducing this character directly upon the viewing screen of the tube in a single operation. More particularly, but not necessarily exclusively, the invention relates to a direct-viewing storage type cathode ray tube having means for producing a cross-sectionally vshaped electron beam conforming to the configuration of a desired character and for storing a charge image of the character and means for utilizing the stored charge image to reproduce a replica of the character on the viewing screen.

In accordance with the foregoing remarks it will be understood that while the invention as described herein is embodied in a storage type cathode ray tube, it is not limited only to utilization in storage tubes but may be employed in any tube of the type having a cross-sectionally shaped character formed beam. A storage tube of the type in which the invention is useful is shown and described in the copending application of the present inventor, S.N. 678,278, filed August 15, 1957, now abandoned, and assigned to the instant assignee. This storage tube comprises means for producing a character writing electron beam, a gun for producing flood electrons, a viewing screen, and a storage screen disposed adjacent the viewing screen to control the ilow of flood electrons to the viewing screen in accordance with charge patterns on the storage surface. The character-shaped writing electron beam is produced by a matrix containing apertures each shaped according to an individual information character. An electron gun for producing an electron beam of sufficient cross-sectional area to cover one character-forming aperture of the matrix is also provided. By suitable deection means the writing electron beam is directed through the aperture of the matrix having the configuration of the character desired to be reproduced. Shaping of the beam, cross-sectionally, thus is achieved by interception of the matrix so that the remaining cross-section of the beam which passes through the aperture has the shape of the character-forming aperture. Subsequently the shaped beam is directed by means of an electron lens and deflecting means to the portion of the storage surface Whereat it is desired to establish a charge pattern having the configuration of the character desired. In general, by secondary emission, the high energy writing electron beam produces a positive charge on the areas of the storage surface on which the beam impinges. This positive charge permits the flood electrons to pass through the storage screen to the viewing screen to produce a continuous visual reproduction of the character.

In the foregoing copending application of the instant inventor the advantages of an electrostatic lens system to convergethe writing beam back toward the tube axis and to focus the beam upon the storage target are taught. Itis further taught that the apertured character-forming iCC matrix may be positioned within an electrostatic converging lens system so as to provide a convergence action on the beam prior to passage thereof through the matrix. In this manner off-axis excursions or deflections of the beam, necessary to achieve the selection of a particular aperture in the matrix, may be limited to the extent actually required due to the oil-axis" disposition of characterforming apertures in the matrix. Limiting such olf-axis excursions tends to reduce pin cushion distortion in an array of visually displayed characters and permits a smaller lens diameter to be employed. Reduction of the electrostatic lens diameter in turn permits the employment of tube necks having a smaller diameter and the possible utilization of a magnetic deflection system to direct a shaped beam toward any particular portion of the storage target for ultimate visual presentation at a corresponding portion of the viewing screen.

While the advantages of the arrangement shown and described in the aforementioned copending application are unquestioned, it has been found that other advantages may be realized by establishing equal or symmetrical electrostatic converging fields on either side of the matrix. Where the converging fields on either side of the matrix are not symmetrical, an aperture lens is formed at each character-forming aperture in the matrix which causes the electron beam to become retracted. This refraction on the part of apertures in a matrix disposed between non-symmetrical converging fields is identified as the aperture lens elect. There is also another diverging action on an electron beam passing through an electrostatic converging lens which occurs as the beam leaves the short, middle electrode of a saddle type lens system which likewise results from the non-symmetrical converging iields of the system. It will be appreciated that off-axis excursions of the beam cannot be minimized to the extent desired so long as the convergence lens system has some diverging action on the beam, no matter how small the diverging action may be. It has been discovered, however, and is a feature of the present invention, that the diverging portion of an electrostatic lens may be eliminated and off-axis excursions of the beam passing therethrough may thereby be completely minimized or reduced to the extent of excursions caused solely by inten-tional olf-axis deflection of the beam.

It is therefore an object of the instant invention to provide an improved cathode ray tube of the type employing an apertured matrix for cross-sectionally shaping an electron beam with an electrostatic converging lens system which establishes symmetrical electrostatic converging fields on either side of the matrix and which limits olfaxis excursions of the electron beam.

These `and other objects and advantages of the invention, as will be more fully explained hereinafter, are accomplished by mounting an electrically conductive apertured character-forming matrix in electrically conducting relation within the short middle lens cylinder of a saddletype electrostatic lens. The usual three-cylinder saddletype lens is thus effectively a two-element lens back-toback with the matrix disposed in the center thereof and having symmetrical electrical converging fields on either side thereof, substantially eliminating any diverging action of the usual saddle-type lens as Well as avoiding the aperture lens effect. By maintaining a positive potential on the short middle cylinder with respect to the outer cylinders, space charge spread of the electrons in the beam V.is advantageously reduced or limited which in turn permits higher beam currents to be employed resulting in an increase in writing speeds. Aberration or distortion vof an array of characters imaged on the viewing screen needed to select the furthest ott-axis aperture in the 'matrix The 'nece's'si'y'rieck size ofthe'tube is`also'n1ini- "erence to the drawings vin which:

Fig. 1 is a sectional, partially schematic view in elevation of a cathode ray tube embodying the instant invention; and

Fig. 2 is a schematic, elevational view of the electrostatic converging lens of the invention.

Referring now to the drawings, there is shown in the sole figure thereof an embodiment of the present invention comprising an evacuated envelope 1 having enlarged bulb portion 2 and a narrow neck portion 3. Disposed in the bulb portion 2 is a storage screen 4 and a viewing screen 5. Also within the bulb portion 2 is disposed a flood election gun 6 which is adjacent the neckportion 3 and facing the storage screen assembly 4. This ood gun 6 is also referred to as the viewing gun. Disposed in the neck portion of the tube in the order named proceeding from the end of the neck portion 3 toward the bulb portion 2 are: an electron gun 7 (also called the writing gun); a first deflection system 8 (also called the aperture- `selection system); an electrostatic convergence lens system 9 in which is disposed an apertured character-forming matrix 10; a compensating plate system 11; and a second deflection system 12.

The writing electron gun 7 includes a cathode 13, a heating element 14 therefor, an intensity grid 15, accelerating electrodes 16 and 17, and a focusing electrode 18. -The heating element 14, one side of which may be connected to the cathode 13, as shown, is connected across a source of potential such as a battery 19. The cathode 13 may be maintained at a potential of about 3100 volts negative with respect to ground by means of a connection to the negative terminal of a battery 20, whose positive terminal is connected to ground. The intensity grid 15 may be maintained at a sutciently negative quiescent potential with respect to that of the cathode13 so that the flow of electrons therefrom is entirely cut off. This is accomplished by a battery 21 which provides a potential of about 100 volts negative, in series with a resistor 22 connected between the cathode 13 and the grid 15.

In order to permit the electrons to flow from the cathode 13, the potential on the intensity grid 15 is decreased until it is only from about 25 to 50 volts negative relative to the potential of the cathode 13. This maybe accomplished by impressing a pulse of suitable amplitude generated by a pulse generator 23 on the intensity grid 15 by means of a connection therebetween through a capacitor 24. The duration of the pulse is for the time necessary to write one character on the storage screen 4.

The accelerating electrodes 16 and 17 are maintained at a potential of about 200 volts positive with respect to ground'by means of a connection to a reference bus 25 which is maintained at the desired potential by suitable i connections to a battery 26. An adjustable potential of about 800 volts positive relative to the cathode 13, or about 2300 volts negative with respect to ground, is maintained on the focusing electrode 18. This is accomplished by means of a connection from the focusing electrode 18 to an adjustable tap 27 of a potentiometer 28 which is,

in turn, connected across the battery 20.

The matrix 10 may be in the form of a thin conductive circular electrode plate approximately one inch in diameter and having a plurality of apertures therein each in -the shape of a particular character to be reproduced. In

a typical installation 64 character-shaped apertures each of which has a height of about 0.022" are arranged to form a square of 8 rows and 8 columns. In order that minimum detlectionof the electron -beam be required,\it

i is preferable that most of the character-forming apertures be disposed as near the ycenter region of the matrix ras f4 possible. The electron beam should be of appropriate cross-sectional area so'asto completely cover only'one aperture in the matrix at a time.

Disposed between the writing electron gun 7 and the electrostatic convergence lens system 9 is a character aperture-selection system Scomprising horizontal deflecting plates 29 and verticaldeeeting plates 30. A quiescent potential of about 200 volts 'positive with respect to ground is maintained on the horizontal and vertical plates 29 and 30 by means of a connection to the reference bus 25 through the isolation resistors 32, 33 and 35, 36, respectively. The horizontal deflecting plates 29 are energized by a horizontal voltage generator 31 connected across the isolation resistors 32 and 33. In a like manner the vertical deflecting plates 30 are energized by a vertical voltage generator 34 connected across the isolation resistors 35 and 36. A control circuit 37, coupled to the grid pulse generator 23 and to the horizontal and vertical voltage generators 31 and 34, is provided to first determine the vertical and horizontal voltages to be produced in order to have the aperture-selection system 8 direct the electron beam through the aperture corresponding to a desired character in the matrix 10 and, secondly, to trigger the pulse generator 23 to permit the electron beam to flow through the aperture for a period of time required for a character to be written on the storage screen 4. It is preferred that there be no flow of writing electrons when changing from one aperture to another in the matrix 10 and that the electron beam does not flow for too long'a period through a single aperture because of the possibility of damage to the storage screen.

An electrostatic convergence system 9 is provided by the metallic lens electrode cylinders 40, 41 and 42 with the apertured character-forming matrix 10 disposed therein as will be more fully described hereinafter. The internal diameter of the lens cylinders 40, 41, 42, 43 and 44 may be approximately 1.4" in a typical installation with a 1.0" over-all diameter matrix. Typical lengths of the lens cylinders 40, 41, 42, 43 and 44 are, respectively, 3.350", 0.60", 3.350, 0.90 and 0.60", spaced approximately 0.10" apart. Not only is a convergence lens action provided by the invention but this is achieved by means of two separate equal converging fields established on either side of the matrix so that this portion of the convergence lens 9 exerts no divergent action whateveron an electron beam passing therethrough. The lens electrode cylinders 43 and 44 constitute in cooperation with the lens cylinder 42 an imaging lens which focuses the matrix 10 on the viewing screen 5. The two end electrodes 401and 42 of the convergence lens system 9, respectively, are maintained at potentials of about 200 volts each and positive with respect to ground by means of connections to the reference bus 25. The short, middle electrode 41 is maintained at a potential of about 2750 volts positive with respect to ground by means of a connection to a tap 45 on a resistor 46 in series with a battery 47. The imaging electrodes 43 and 44 are maintained at a potential of about 2900 volts negative and 200 volts positive, respectively, with respect to ground. The electrode 43 is connected to a tap 48 on the resistor 46 in series with a battery 49. The electrode 44 is connected to the reference bus 25. The function of the lens system 43 and 44 is to primarily focus the matrix at the storage screen and to further converge the electron beam toward the tube axis.

It will be appreciated that the writing electron beam which is converged toward the tube axis by the convergence lens will have a trajectory which is at an angle with respect to the axis of the tube. In order to center each character within a single predetermined elemental area on the storage screen, it is necessary that the electron beam enter the dellecting system substantially along the central axis thereof. Hence, in accordance with the teachings of the aforementioned U.S. Patent 2,769,116 to N. J. Koda et al., a compensating plate system 11 is disposed between the electrostatic convergence lens 9 and the storage target selection detlectin'g means 12. This compensating plate system comprises a iirst pair `49 of compensation plates and a second pair 50 of compensation plates. The rst pair of plates which compensates for the vertical olf-axis component of the writing beams trajectory is maintained at a quiescent potential of about 200 volts positive by means of connections to the taps S1 and 51' on the isolation resistors 35 and 36, respectively, in series with the vertical voltage generator 34 which also supplies vertical deflection voltages to the aperture-selection system 8. The second pair of compensating plates which compensates for the horizontal olf-axis component of the trajectory of the electron beam is maintained at a quiescent potential of about 200 volts positive by means of connections to the taps 53 and 53' on the isolation resistors 33 and 32, respectively, in series with the horizontal voltage generator 31 which likewise supplies horizontal deflection voltages to the yaperture-selection system 8. It will be understood that this arrangement is employed so that the action of the compensation plates may be commensurate with the action of the aperture-selection deflecting system.

The storage target selection system 12, comprising a second deflection system in the tube, includes the horizontal deection plates 55 and the vertical deflection plates 56 disposed about the path of the electron beam as shown. Both the horizontal and vertical deilection plates are maintained at a quiescent potentialof about 200 volts positive with respectto ground by means of connections to the reference bus 25 through the isolation resistors 57, `58 and 59, 60, respectively. The horizontal deflection plates 55 are energized by a horizontal deflection voltage generator 61 connected thereto across the isolation resistors 57, 58 through the capacitors 63 and 64. In a like manner, the vertical deflection plates S6 are energized by a vertical deflection voltage generator 62 connected thereto across the isolation resistors 59, 60 through the capacitors 65 and The viewing or ood gun 6 in conjunction with the electrodes 67, 68 and 69 produces a broad beam of flood electrons that is directed uniformly over the entire area of the storage screen 4. The flood gun`comprise's a cathode 70 surrounded by an intensity grid 71 and an annular electrode 72 disposed in front of the intensity grid. The cathode 70 is operated at ground potential by means of a connection thereto while the intensity grid 71 is maintained at a potential of about l0 volts negative with respect to ground which is accomplished by means of a connection from the grid 71 to an adjustable tap 73 of a potentiometer 74. The potentiometer 74 is connected Aacross ya battery 75. An intermediate point of the battery is connected to the ground so that both positive and negative potentials are available from the potentiometer. The annular electrode 72 is maintained at a potential of about 200 volts positive with respect to ground by means of a 'connection to the reference bus 25. The potential maintained on the intensity grid 71 depends upon the desired ood beam diameter and current.

VDisposed concentrically about the inner periphery of lthe bulb portion 2 are the electrodes 67 and 68 whose purpose is to collimate the flood electrons. These electrodes may be composed of a conductive coating, for example, by painting the inside of the bulb portion with a colloidal solution of carbon such as aquadag The electrode 67, nearest the ood gun 6, is maintained at a potential of about 200 volts positive with respect to ground 'by means of a connection to the reference bus 25. The other electrode 68, furthest from the flood gun 6, is maintained at a potential of about 50 to 150 volts positive with lrespect to ground by means of a connection to an adjustable tap 77 on a potentiometer 78 which is connected across a battery 79 which has an intermediate terminal connected to ground. Also assisting in collimating the flood electrons is the annular electrode 69 which is disposed adjacent the storage target and extends toward the ilood gun within the space defined b`y' the collimting electrode 68. The electrode 69 is maintained at a potential of about 250 volts positive with respect to ground by a connection to an adjustable tap 76 on the potentiometer 78.

Disposed in the bulb portion of the envelope are the storage screen 4 and the viewing screen 5. The storage screen is of the type shown and described in U.S. Patent 2,788,467 to H. M. Smith and comprises a metal ring 80 welded to one side of the periphery of an electroformed nickel screen 81 which serves as a contrast control'igrid. The nickel screen 81 is of the order of 0.00052 to 0.001 inch thick and has yabout 250 meshes per inch. A coating of dielectric material (not shown) about 0.002 inch thick and possessing secondary electron emission characteristics,

is disposed uniformly over the side of the nickel screen 81 without overhanging or overlapping any of the meshes thereof. A suitable dielectric material is talc. A collector electrode 83 comprising a thin conductive screen 85 is disposed over the surface of the dielectric coating 82. The collector screen 85 has Iabout 100 to 200 meshes per inch for example. The meshes of the collector screen 85 are disposed at an acute angle with the meshes of the contrast control screen 81 so as to minimize moire effect. The contrast control screen 81 is maintained at a potential of about 10 volts negative with respect to ground by means of a connection to the tap 86 on the potentiometer 78. The collector screen 85 is maintained at a potential of the order of volts positive with respect to the ground by means of a connection to a tap 87 on the potentiometer 7 8.

Disposed adjacent to and behind the storage screen on the inner surface of the end of the enlarged bulb portion 2 of the tube envelope is the viewing screen 5. The viewing screen comprises a conductive transparent layer 88 and a thin uorescent layer 89. The conductive transparent layer 88 may be disposed directly on the glass surface of the envelope and may consist of a layer of stannous oxide. This transparent conductive layer is maintained at a high positive potential so as to accelerate electrons at a sufficiently high velocity to excite the fluorescent screen upon impingement therewith. This potential which may be of the order of from 5000 to 10,000 volts positive is provided by a connection to the battery 90.

Referring now to the drawings and particularly Fig. 2, the metallic apertured-character-forming matrix 10 is mounted inthe saddle electrode cylinder 41 so as to be in electrical contact therewith as by welding, for example.

As shown, a ring mount 92 may be provided inside the electrode cylinder 41 and secured thereto by welding, with the matrix plate `10 being spot-welded, for example, to the ring mount 92. The ring 92 may have an over-all diameter of 1.375, for example, and be provided with a. recessed portion having va diameter of 1.0 to receive the matrix 10 and to provide an annular support surface of about 0.22" for the matrix while leaving unobstructed area of about 0.56" Vof the matrix which area comprises principally the character-forming apertures. The eiect of locating such an electrical shorting member across the saddle electrode 41 is to change the lens from a onceld to a two-field lens thus providing substantially equal converging lens tields back-to-back on either side of the matrix. As shown in the drawings by the dotted lines 91, the lines of force in each eld now exert only a converging action on a beam travelling therethrough. Without this arrangement, as in the prior art, the converging lens, in addition to an over-all convergence force, tends to exert a diverging force on the beam. Since in the present embodiment, the matrix 10 is located in the approximate centei of the lens system 9, the matrix cannot be imaged onto the viewing screen. Hence, a second saddle lens is provided by means of the electrode cylinders 43 and 44 in cooperation with the electrode in order to give the desired magnication of the aperture characters at the screen. The location of the imaging lens system 43 and e, aaa-eee 7 44. is primarily determinedJ by the magnification; desired rather than by the size of the beam-bundle as was done heretofore. Hence, low magnificationof the mateix' may befemployed resulting in a higher beam current density at the viewing screen than heretofore.

It will be appreciated that the maximum excursion of the beam off the tube axis is no more than required by the'disposition of the maximum off-axis apertures in the matrix. Hence, aberration from the imaging lens, which increases as the cube of the off-axis vbeam excursion radius, is minimal for the radius of the matrix employed. Since the diameter of the tube neck is dictated by the diameter of the convergence lens, by the present invention, this diameter can be acceptably small' enough to permit the use of a relatively small-diameter, minimal distortion magnetic deflection yoke in-place of the electrostatic defiection plates 55 and 56 employed forv target selection shown in Fig. l.

Another advantage of the invention lies in the fact that the high positive potential atthe matrixtends to reduce the effect of space charge forces in the beam with the result that higher beam currents may be employed. A tube employing the arrangement of the invention may be operated at three times the beam current of a tube Without the invention, thus resulting in faster writing times and writing speeds. ln one tube such as shown in Fig. 1, it was found that the writing time per character was 20 microseconds and that writing speeds of 40,000 inches per second were obtainable.

In operation, upon the application of suitable horizontal and vertical defiecting voltages in response to the control circuit 37, the aperture selection deflection system 8 directs the electron beam from the writing electron gun 7 toward the particular aperture in the matrix which corresponds to the character desired to be written. As the electron beam enters and traverses the electric field of the electrostatic convregence lens system 9, it is bent or turned back toward the tube axis and follows a path which may be converging toward the tube axis. Upon passing through the predetermined aperture in the matrix, the electron beam acquires a cross-sectional shape corresponding to the character-forming aperture. After passing through the matrix, the beam is further converged toward the tube axis by the action of the second electrostatic lens system 42, 43,'and 44 which also focuses the matrix upon the storage screen 4. Upon reaching the compensating plate system 11, the beam is again redirected thereby so as to be substantially coincident with the axis of the tube. Thereafter the target deection system 12, in response to the control circuit 37, directs the beam to the particular portion of the storage screen 4 on which it'was desired to store the character.

The character-writing electron beam upon impinging on the storage surface causes the liberal emission of large numbers of secondary electrons which are attracted to the collector screen S5. In this manner a pattern of positive (or less negative) charges is formed on the storage surface, the pattern corresponding to the shape of the beam and hence the aperture which formed the beam. The positively or less negatively charged portions on the storage surface are then rendered visible by the action of flood electrons from the viewing gun 6. The flood electrons penetrating the storage surface only through the positively or less negatively charged areas thereon, are thereafter accelerated toward the viewing screen to produce a visible image corresponding to the shape of the storage pattern.

It will thus be appreciated that according to the instant invention convergence of the deflected writing electron beam back toward the tube axis is `achieved in two steps and with two distinct converging lens fields rather than in one as-has been the practice heretofore. Thus an electron beam which must be initially deflected off the tube axis in order to reach the selected aperture on the character forming matrix is prevented from diverging off the tube axis to a greater extent than is necessary in order to reach the particular aperture/desired.: The lens arrangement of the invention limits off-axis excursion of the beam and eliminatesy any incidental diverging forces on the beam characteristic of prior convergence llens arrangements; the lens arrangement of the invention also returns the beam back toward the tube axis and images the `matrix at the screen. It will thus be understood that the maximum offaxis excursion of the beam is primarily determined by the off-axis disposition of apertures in the character-form ing matrix. Hence the diameter of the electrostatic lens system for converging the beam is minimal, being` determined primarily by the dimensions of thecharacter-formingl matrix and the disposition of the apertures therein. For example, in a storage tube having a character-forming matrix with the dimensions referred to hereinbefore (i.e., .300 inch square), an electrostatic lens field established according to the invention need have a diameter of only 11/2 inches.

It should be understood that while the embodiment of the invention described employed the apertured characterforming matrix as thefelectrical shorting element in the middle electrode cylinder 42, any electron permeable electrically conductive element may be employed for this purpose. In such an embodiment the charactenforming apertured matrix might be of electrically nonconducting ma` terial, for example, disposed substantially in the same position in the middle electrode cylinder with the electron permeable electrically conductive element or membrane disposed adjacent the matrix and across the lens cylinder. It should also be appreciated that the invention may be employed in cathode ray tubes of the non-storage type to equal advantage.

There thus has been described an improved cathode ray tube of the type employing an apertured matrix for forming information characters upon a target having a lens system of minimal diameter and distortion. In addition to achieving these results, the invention also permits the attainment of a higher beam current, character writing times and speeds, and the selection of a more optimum magnification of the apertured-character-forming matrix on the viewing screen;

What is claimed is:

l. An electrostatic lens system for a cathode ray tube having an apertured matrix therein wherein an electron beam is passed through said apertures to produce a particular cross-sectional configuration, said lens system comprising means Ifor generating rst and second symmetrical electrostatic lenses about said apertured matrix and means for generating a third electrostatic lens on the side of said second electrostatic lens farthest from said matrix to electrostatically focus said matrix on a plane spaced a predetermined distance therefrom.

2. A cathode ray tube comprising means for producing an electron beam along a predetermined axis, an apertured matrix for cross-sectionally shaping said electron beam upon passage thereof through said apertures, means for directing said electron beam off of said predetermined axis to a pre-selected one of said apertures, means for establishing symmetrical electrostatic fields on both sides of said matrix for pre-converging said beam towards said matrix and for post-converging said beam towards, said predetermined axis after passage of said beam through said matrix, means for establishing an additional electrostatic eld to focus said matrix on a target electrode, and means to direct said cross-sectionally shaped beam to a predetermined portion of said target electrode.

3. A cathode ray tube comprising an electron gun for producing an electron beam; first, second, third and fourth conductive cylindrical elements disposed along a common axis in the order named in proceeding away from said electron gun; a matrix having a plurality of information character-forming apertures therein disposed transversely across the inside of said second conductive cylindrical element; means for successively directing said electron beam toward selected apertures in said matrix thereby 9 to shape the cross-section of said electron beam into the configura-tion of the corresponding character; means for applying a iirst direct-current potential to said second conductive cylindrical element; means Ifor applying a vsecond and a third direct-current potential dilerent from said first direct-current potential to said iirst and third conductive cylindrical elements, respectively, for producing a first electrostatic lens to pre-converge said electron beam onto said matrix and a second electrostatic lens to postconverge said electron beam subsequent to the traversal thereof through said matrix; a target element spaced from the extremity of said fourth cylindrical element that is farthest from said electron gun;v means for applying a fourth direct-current potential ditierent from said third direct-current potential to said fourth cylindrical element for producting a third electrostatic lens intermediate said second electrostatic lens and said target element for imaging'the information character-forming apertures of said matrix on said target element; and means disposed intermediate said -fourth cylindrical element and said target element for successively directing said character-shaped electron beam towards selected areas of said target element.

4. The cathode ray tube as deined in claim 3 wherein the diameters of said first, second and third conductive cylindrical elements are substantially equal, the lengths of said first and third cylindrical elements are no less than one diameter and the length of said second cylindrical element is from 0.4 to 0.8 diameter thereby to locate said first and second electrostatic lenses immediately adjacent to opposite sides of said matrix.

5. The cathode ray tube as dened in claim 3 wherein the diameters of said trst, second and third conductive 10 cylindrical elements are substantially equal, the lengths of said first and third cylindrical elements are substantially equal to three diameters and the length of said second cylindrical element is substantially equal to 0.6 diameter thereby to locate said first and second electrostatic lenses immediately adjacent to opposite sides of said matrix.

6. The cathode ray tube as defined in claim 3 wherein the diameters of said first, second and third conductive cylindrical elements are equal and said second directcurrent potential is equal to said third direct-current potential whereby said first and second electrostatic lenses are symmetrically disposed about said matrix. f

7. The cathode ray tube as defined in claim 6 wherein said first direct-current potential is substantially more positive than said second and third direct-current potentials thereby to accelerate said beam in the region adjacent said character-forming matrix whereby the characterforming apertures of said matrix allow an increased portion of the current constituting said electron beam to pass therethrough.

References Cited in the file of this patent UNITED STATES PATENTS tion, Part 10, vol. 3, Nos. 6-10, 1955, pages 55 to 61. 

